Aluminum tubes have been and are used as the containers for various contents that are in a paste state. Each aluminum tube has a cylindrical barrel and a mouth continuing from one of opposite ends of this barrel through a shoulder. The other end of each tube is a skirt that will be kept open while the tube whose mouth has been closed with a cap is subsequently charged with the content. Finally, the rim of such a skirt will tightly be folded back to seal the bottom of this tube so that the content is isolated from ambient air. The content will thus be protected from any adverse effect of air or humidity so as to be free from degeneration or deterioration during a long-term storage. Generally, the upper portion of cylindrical barrel of each tube gradually decreases its diameter towards the mouth, due to presence of the substantially frustoconical shoulder. In other words, diameter of the mouth is considerably smaller than that of said barrel.
Materials of such tubes are usually aluminum, tin, lead or alloys thereof. After having depressed the barrel of each metallic tube to extrude its content through the mouth, it will maintain its depressed configuration, lest any amount of ambient air causing degeneration or deterioration of the content should intrude into it. Therefore, such metallic tubes easy to manufacture and processing and convenient to carry are widely used as the containers for pastes of various pharmaceuticals, hairdressings, hair dyes, cosmetics, foods or adhesives.
It is to be noted that some contents will cause corrosion of metallic tubes filled with them. In this case, an anti-corrosion resin layer or the like has been formed on the inner periphery of each tube by the electrostatic powder coating system.
Practically, a spray gun will be placed in the tube through its open skirt, and a fine resin powder is blown into the tube together with air so as to stick to the inner periphery. Thereafter, the tube will be heated to melt the layer of resin powder and then cooled down to form a solid resin membrane on said periphery.
However in such a powder coating method, the inner periphery of tube shoulder is prone to receive a very excessive amount of the resin powder, failing to spread it uniformly all over the inner peripheries of mouth, shoulder and barrel of each tube. As a result, an excessively thick resin membrane on the inner periphery of shoulder has often hindered smooth extrusion of content. In detail, such a shoulder will resist its depression, making it difficult to squeeze a residual amount of the content stagnant around the shoulder, out of such a tube.
In addition, such a molten and thickened resin membrane formed on the inner periphery of shoulder will show a remarkable secondary shrinkage when it solidifies, probably causing exfoliation of the other lining portion sticking to the inner periphery of mouth.
In some cases, a certain region of inner periphery of the skirt that is to be folded back and sealed should be masked from the resin powder. It has however been difficult to selectively mask only such a skirt region, because the resin powder has been blown in through said skirt.
The prior art powder coating method has therefore been carried out in such a manner that ejection pressure and flow rate of the resin powder would be regulated taking a view of the coating being formed. If a moderate and proper amount of resin powder is applied to the shoulder, then the barrel would be provided with an insufficient quantity of said powder, probably producing a number of pinholes in such a thinned resin coating. On the contrary, if a moderate and proper amount of resin powder is applied to the barrel, then the shoulder would be provided with a very excessive quantity of said powder. Similarly to such a dilemma, a proper and uniform application of resin powder to the barrel has often failed to provide a skirt opening reliably coated with a reduced amount of said resin.
The prior art powder coating apparatuses have undesirably produced container tubes with inner peripheries coated with resin layers of varied thickness, thereby lowering yield of satisfactory products coming up to the standard, even if ejection pressure and flow rate would have been controlled. Flow passages and spray gun included in each of those prior art apparatuses have been likely to be clogged with resin powder, and therefore improvement of them has been required.